1. Field of the Invention
The present invention relates to a flight control system for an airplane, particularly to a drone that is capable of holding a fixed radius, a fixed altitude and a fixed speed when making a steep turn for use of an unmanned target plane to confirm and evaluate a target tracking capability of a missile or other system.
2. Related Background Art
An altitude, a vertical flight path angle and a speed are controlled through changing the flight path angle by typically operating a pitch axis control device such as an elevator, etc. enough to cause rotation about a pitch axis, thus changing an elevator angle, increasing or decreasing a lift along with the change and thereby obtaining an acceleration in a perpendicular direction of the airplane. For example, JP-A-61-196896 discloses a flight control under which a turn is carried out by holding a designated bank angle while holding the altitude by this system. FIG. 8 shows this known system.
Referring to FIG. 8, an attitude sensor 12 detects an attitude of an airplane 10 to be controlled. An altitude sensor 14 detects an altitude thereof. Outputs of these detections are input to a flight control apparatus 16. The attitude sensor 12 detects a bank angle .phi. and a pitch angle .theta. of the airplane 10. The bank angle .phi. is given as a feedback quantity to a bank angle control system, and the latter pitch angle .theta. is given as a pitch angle correction quantity to an altitude control system. The flight control apparatus 16 includes a roll axis control device 18 and a pitch axis control device 20. The roll axis control device 18 obtains a roll axis steering angle command as a manipulated variable to reduce a deviation .DELTA..phi. (=.phi.*-.phi.) of the bank angle .phi. from a target bank angle .phi.* obtained by a comparing unit 22 to zero. A roll axis is controlled in dependency on the roll axis steering angle command through a roll axis maneuvering device 26, e.g., an aileron. The pitch axis control device 20 obtains a pitch axis steering angle command as a manipulated variable to reduce a deviation .DELTA.H (=H*-H) of a real altitude H from a target altitude H* obtained by a comparing unit 24 to zero with reference to the pitch angle .theta.. A pitch axis is controlled in dependency on the pitch axis steering angle command through a pitch axis maneuvering device 28 with reference to the pitch angle .theta., e.g., an elevator.
The above publication further discloses the system, wherein a feedback gain of the bank angle control system is increased when the real altitude decreases under the target altitude which might happen during a steep turn while controlling the altitude by controlling the pitch axis as well as controlling the bank angle by controlling the roll axis. The bank angle is thereby decreased to enhance the lift in the perpendicular direction, and an altitude recovery function is thus enhanced.
The vertical direction of the airplane 10, i.e., a lift axis Q is close to a perpendicular axis, i.e., an altitude direction Z as illustrated in FIG. 9 in a wing level straight flight of the airplane and a gentle turn at a small bank angle .phi.. It is therefore possible in the prior art to obtain a perpendicular component .DELTA.Lz for acquiring an acceleration with respect to a perpendicular lift component Lz to sustain an airplane weight, i.e., a lift component in the altitude direction Z by increasing and decreasing a lift increment .DELTA.L for a lift L in the direction of the lift axis Q.
The lift axis Q is inclined to close as horizontal as illustrated in FIG. 10 on the occasion of high maneuvering as in case of a steep turn taking a large bank angle .phi.. Hence, there must be largely increased and decreased lift increment .DELTA.L in the direction of the lift axis Q corresponding to the perpendicular lift component .DELTA.Lz for obtaining the acceleration in the altitude direction Z to control the altitude. Note that a horizontal component Lh of the lift L acts as a centripetal force, and a horizontal component .DELTA.Lh of the lift increment .DELTA.L acts as an increment of the centripetal force to change a turn radius in FIG. 10.
First, when the control device has to be operated larger than in the straight flight or the gentle turn when performing the high maneuvering as in case of the steep turn, e.g., taking the large bank angle .phi., the same control effects can not be obtained. Further, a high control accuracy is not necessarily required only for the altitude and the speed control in case of the turn. The turn acceleration of an unmanned plane is required to be held precisely to a predetermined value when turning and evading the unmanned target plane for the purpose of evaluating a target tracking capability of, e.g., for evading a missile pursuit. It is also required that the turn around a ground target object must be carried out while keeping the turn radius constant in the flight control of the airplane observing the ground target object.
The turn acceleration proportional to the lift largely changes, and further the turn radius is liable to change because of the turn centripetal force defined as a horizontal component of the lift being largely varying in the prior art system requiring largely increasing and decreasing the lift to control the altitude. Accordingly, second, it is difficult to make holding the altitude and the flight path angle compatible to holding the turn acceleration and the turn radius in the prior art system.
Next, there will be considered the control accuracy in a steady state of being stabilized at a fixed bank angle, altitude and speed.
For the purpose of attaining the steady turn at the turn acceleration set by the prior art control system as shown in FIG. 8, a desired turn acceleration is attained by inputting the bank angle command determined by the following relational formula: EQU N=1/cos.phi. (1)
where N is the turn acceleration in the steady turn, and .phi. is the bank angle. In this case, if the attitude sensor 12 has a detection error, the bank angle to be realized is different from the command value, corresponding to this error, and it follows that the turn acceleration to be realized has an error for the set value. If there is an angular error on the order of 2.0.degree. when making a 2 G turn wherein N=2.0, and .phi.=60.0.degree., what is in fact realized is: .phi.=62.0.degree., and N=2.13. In this case, the error in the turn acceleration N is not so large. If the target is: N=5.0, and .phi.=78.46.degree., however, what is in fact realized is: .phi.=80.46.degree., and N=6.04. The error in the turn acceleration N exceeds 1 G, and a large error in the turn acceleration is caused for a slight error in the bank angle as the turn gets steeper.
Accordingly, third, it is difficult to carry out the steep turn at a high accuracy according to the prior art system, wherein the turn acceleration is designated.
Furthermore, it is known that the altitude control by the pitch axis control device becomes hard in a region called a backside in which the speed of the airplane is slow enough to be under about 1.5 times the stalling speed although there is some latitude depending on the characteristics of each airplane. It is assumed that the airplane enters the backside at a higher speed especially in the steep turn than in the straight flight and the gentle turn. Accordingly, fourth, there is such a problem that the steep turn is hard to carry out at a low speeds with difficulty controlling the altitude in the prior art control system.
The recovery function lowering with only the increase and decrease in the lift when the altitude goes down in the conventional manner, is enhanced by reducing the inclination of the lift axis as a bank angle decreased and by incrementing the perpendicular component of the lift in JP-A-61-196896. According to this system however, the two kinds of control functions through the pitch axis and the roll axis and are applied to holding the altitude, and therefore can not correspond to other control purposes such as holding the turn acceleration and the turn radius. Further, the altitude is controlled by controlling the pitch axis, and hence the difficulty for use in the backside remains unchanged.
That is, the prior art cited herein gives a solution for the first problem described above but can not provide any solutions for the second, third and fourth problems.